This invention relates to a method for making corrosion-resistant ceramic shapes. More specifically, this invention relates to a method for making corrosion-resistant ceramic shapes having a tungsten metal coating.
There is a need for suitable materials for use as containment vessels which are capable of withstanding the highly corrosive molten metals and salts, such as are used in pyrochemical methods for nuclear fuel reprocessing. These solutions, which include molten alloys such as molten magnesium, copper and uranium or molten salts such as magnesium chloride, sodium chloride and potassium chloride are all very corrosive and difficult to contain, particularly at reprocessing temperatures which may go as high as 700.degree. to 850.degree. C.
One of the few materials which is capable of withstanding the corrosive attack of molten metals and salts such as these, is tungsten. However, the use of tungsten in corrosion-resistant hardware for containment, piping, agitation or other purposes is severely limited by its poor fabricability. Material properties of tungsten include a high melting point, high ductile to brittle transition temperature, high hardness and low ductility, all of which make casting or hot working and machining of large or complex shapes of tungsten virtually impossible. The welding of tungsten is also generally precluded by poor strength in the highly recrystallized heat-affected zone, poor penetration of the base metal by the weld, thermal shock problems associated with heating and cooling in the brittle temperature region, and difficulties in stress relief after welding. Consequently, the use of tungsten is largely limited to components of relatively small size and simple geometry.
Ceramic materials such as alumina may be fabricated into larger and more complex shapes, such as tubing and crucibles, with higher reliability and lower cost than tungsten metal. However, for use in highly corrosive environments, ceramic components must have a suitable protective coating.
While the application of a tungsten metal coating on ceramic shapes by methods such as vapor deposition is possible, it is difficult to accomplish and expensive because it requires that the substrate be heated and maintained at a high temperature in order for the metal to adhere. Furthermore, tungsten coatings supplied in this manner have a unidirectional grain structure which invites problems of grain boundary attack by the corrosive substance which could rapidly penetrate through the coating and destroy the ceramic shape. In Tungsten Metallizing Alumina-Yttria Ceramics, LA-6705-MS (1977), by Cowan, R. E. and Stoddard, S. D. the metallization of alumina bodies by the addition of small amounts of yttria to the alumina before firing is described. The tungsten is applied, as a slurry of finely divided powder in a water vehicle containing glycerine and a commercial electrolyte, to the surface of the alumina shape and fired. Although the coating is suitable for the intended purpose, i.e. as a brazing medium for joining two or more shapes, it is discontinuous and porous and therefore, will not protect the ceramic substrate from a corrosive environment.